In the past niobium-titanium, niobium-tin and vanadium-gallium have been substantially employed as superconductor materials, but to date each is saddled with some limitations. Niobium-tin and vanadium-gallium superconductors are largely limited in their applications because both comprise compounds normally formed by reaction of the respective materials in situ and the reaction product compounds tend to be quite brittle. This brittleness tends to seriously limit further fabrication and even limits the ability of the superconductor to be bent or wound through the relatively small diameters sometimes required to wind magnets. Noibium-titanium alloys, by comparison, when used in certain matrix material are not quite as limited. For instance, noibium-titanium superconductor in a copper matrix has seen substantial application although rapid magnetic field changes have to be avoided since they can produce instabilities in the superconductor, which can generate heat and the resultant deterioration of the superconducting state.
A desirable superconductor composite should generate minimum heat even in rapidly changing magnetic fields and have components which will dissipate any heat which might be generated without adversely affecting electrical performance. The superconductor composite desirably includes a stabilizing member which should have very low electrical resistivity at superconductor operating temperatures, which member can provide an alternate more or less low resistance path for electric current if a region of resistance is encountered in the superconducting member. Additionally, a desirable multiple strand superconductor composite should have a matrix of normally conductive material, or material which has relatively high electrical resistivity at superconductor operating temperatures, to help reduce inductive or eddy current coupling losses between or among the individual superconducting members and minimize the attendant generation of heat. Plastics and other nonconductors satisfy the resistivity requirements for the matrix but lack sufficient strength and ductility required for drawing of fine wire composites comprising multiple superconductor strands.
High purity aluminum used as a stabilizer and aluminum alloy used as a matrix material could provide the needed properties for stabilizer and matrix. High purity aluminum provides low electrical resistivity at superconducting temperatures typically having an electrical resistivity about 10 times less than copper at superconducting temperatures. With respect to the matrix material, aluminum alloy offers advantages because of relatively high electrical resistivity at superconducting temperatures and of good strength, ductility and fabricability. Further, the combination of high purity aluminum as a stabilizer and the aluminum alloy as matrix material in a superconductor offers another advantage in that their respective densities are much less than material commonly used in superconductors. For instance, aluminum has a density of about one-third that of copper and since their respective strength levels can be approximately equal, aluminum can provide a strength-to-weight ratio in the order of 3 to 4 times greater than copper. Strength and weight are important when both Lorentz forces and centrifugal forces are encountered such as in rotating machinery applications, for example power generators. The combination of high purity aluminum and aluminum alloy is beneficial in yet another way when compared to copper; it has a smaller heat capacity which results in energy saving in thermocycling from ambient to cryogenic temperatures. Additionally, aluminum has much greater thermal conductivity at superconducting operating temperatures, when compared to copper and many other materials, and thus allows faster dissipation of any heat that may be generated to the surroundings.
Regardless of these potential advantages, aluminum, especially high purity aluminum, has been limited in its use because of fabricating problems due to its softness. Under conventional practices, in drawing a superconductor having a soft aluminum stabilizer, a drawing problem known in the art as a "flowing phenomena" occurs. That is, the soft aluminum stabilizer has been difficult, if not impossible, to co-reduce with the other components of the superconductor composite, principally the superconducting elements, with consistently satisfactory results. This problem severely limits the use of aluminum, especially high purity aluminum, or any other relatively soft material used as a stabilizer in conventional superconductor composites. Also, under current fabrication practices for conventional superconductor composites, such as annealing, and particularly the precipitation heat treating of the superconductor members, alloying constituents of the matrix can migrate to the stabilizer often seriously increasing its resistivity and diminishing its stabilizing effect which leads to impairment of the composite superconductor performance.
This invention overcomes these problems of using high purity aluminum or a relatively soft stabilizing material in superconductor composites by providing an improved superconductor composite and a method of making the same.